Issuing devices to issue a medium for authenticating a person, such as a license and an ID card, are known. An issuing device prints various information on a printable base material, such as paper, plastic, to issue a personal authentication medium. In addition, in order to prevent forgery, there are a medium on which various security printing has been performed, and an issuing device to perform various security printing.
There is a method to embed a sub image (sub information) into a main image (main information) with a digital watermark. However, in such a method, there is a possibility that the image quality of the main image deteriorates. It has been desired to embed the sub image without deteriorating the image quality of the main image.